Grid/Grid.php
protected function processFilters($permanent = true)
    {
        foreach (($permanent ? $this->permanentFilters : $this->defaultFilters) as $columnId => $value) {
            /* @var $column Column */
            $column = $this->columns->getColumnById($columnId);

            if ($permanent) {
                // Disable the filter capability for the column
                $column->setFilterable(false);
            }

            // Convert simple value
            if (!is_array($value) || !is_string(key($value))) {
                $value = ['from' => $value];
            }

            // Convert boolean value
            if (isset($value['from']) && is_bool($value['from'])) {
                $value['from'] = $value['from'] ? '1' : '0';
            }

            // Convert simple value with select filter
            if ($column->getFilterType() === 'select') {
                if (isset($value['from']) && !is_array($value['from'])) {
                    $value['from'] = [$value['from']];
                }

                if (isset($value['to']) && !is_array($value['to'])) {
                    $value['to'] = [$value['to']];
                }
            }

            // Store in the session
            $this->set($columnId, $value);
        }
    }
protected function processFilters($permanent = true)
    {
        foreach (($permanent ? $this->permanentFilters : $this->defaultFilters) as $columnId => $value) {
            /* @var $column Column */
            $column = $this->columns->getColumnById($columnId);

            if ($permanent) {
                // Disable the filter capability for the column
                $column->setFilterable(false);
            }

            // Convert simple value
            if (!is_array($value) || !is_string(key($value))) {
                $value = ['from' => $value];
            }

            // Convert boolean value
            if (isset($value['from']) && is_bool($value['from'])) {
                $value['from'] = $value['from'] ? '1' : '0';
            }

            // Convert simple value with select filter
            if ($column->getFilterType() === 'select') {
                if (isset($value['from']) && !is_array($value['from'])) {
                    $value['from'] = [$value['from']];
                }

                if (isset($value['to']) && !is_array($value['to'])) {
                    $value['to'] = [$value['to']];
                }
            }

            // Store in the session
            $this->set($columnId, $value);
        }
    }
Store permanent filters to the session and disable the filter capability for the column if there are permanent filters.